It is known from the state of the art, for example from WO 2003/023536 A1, WO 2009/060000 A1, WO 2009/063053 A1 and EP 0940738 A2, to use radio units in plants of process automation technology for easier servicing of the field devices. There are solutions where the field devices have integrated radio units from the start. However, also known is the retrofitting of field devices with radio units, in order that the field devices can subsequently be operated wirelessly.
The energy supply of the field devices as well as the radio units occurs, in such case, for example, via a current loop, to which one or a number of field devices and/or one or a number of radio units are connected. As regards hardware, these electrical current loops are, in such case, a two conductor or a multiple conductor arrangement. The supply of the electrical current loop with electrical energy occurs, in such case, for example, via a supply device, such as, for example, a power supply, which delivers the electrical voltage required for setting an electrical current value in the current loop. If the electrical energy present in the current loop does not suffice to supply all field devices connected to the current loop, it is also possible to provide a separate voltage supply via an additional supply device for individual field devices or a radio unit. For this purpose, it is known, for example, from the state of the art, for example, DE 102004020393 A1, to integrate a battery into the radio unit so that the radio unit and a field device connected thereto do not, in given cases, need to be supplied with electrical energy from the current loop.
In industrial plants, a number of supply devices are often required due to the number of installed field devices, in order, on the one hand, to assure communication via the current loop and, on the other hand, to assure the operational capability of the field devices connected thereto. These supply devices provide a supply voltage, which serves, in each case, for operating the connected field device. For the field device, however, it is not discernible, from which voltage source the supply voltage is taken, or in which state the voltage source is. Therefore, the field device cannot be matched to the type of voltage source and/or its state, for example, to make life lengthening operational settings.